There is a big price gap between GB games CIB and loose. Even with common titles. If it's an overly popular game, the gap got even bigger.
I'll echo everyone else here, $45 isn't bad if it's a game you grew up with and have an itch to play. I found it for $40 w/shipping currently listed, and all the cart only copies (no box, no manual) finished for $30 and under.
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NintendoAge's Resident Tony Hawk Pro Skater Collector